Composition and method for use with ceramic matrix composite t-sections

ABSTRACT

A composition is for use with fabricating a ceramic composite stiffening member that defines a T-section. The stiffening member includes a web portion, at least one flange portion, a radius region disposed between the web portion and the at least one flange portion, and a skin member that is secured to the at least one flange portion and the radius region. The composition is a pliable mixture of refractory particles, plasticizers, silica-yielding polymers, and solvents that is applied along the radius region.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. Utility application Ser. No.10/720,807, filed on Nov. 24, 2003, and entitled “Composition and Methodfor Use with Ceramic Matrix Composite T-Sections”, the disclosure ofwhich is incorporated as if fully rewritten herein.

FIELD OF THE INVENTION

The present invention relates generally to a composition and method foruse with ceramic matrix composite stiffeners. Specifically, the presentinvention relates to a composition and method for use with ceramicmatrix composite stiffeners having T-sections that provides betterconsolidation of ceramic material plies during lamination.

BACKGROUND OF THE INVENTION

Composite materials comprising laminated plies of fabric in a resinmatrix are often used due to their high strength to weight ratio.Fastening two composite parts together, however, is often troublesome.For example, when one composite part is attached to another compositepart, bolts and/or rivets may be used, but such fasteners add weight,increase fabrication cost, and often contribute to local failure modesbetween the individual plies of the laminate composite.

Composite aircraft stiffeners, typically comprised of carbon matrixmaterial, are used to reinforce thin composite structures such as wingand fuselage skins and bulkhead webs. Other composite constructions,such as ceramic matrix composites (CMCs), provide similar structuralsupport in regions of the aircraft that are subjected to hightemperature, such as components associated with engine exhaust, andpossibly the engine itself. Due to the frequent mutually perpendicularshape, such stiffeners are often referred to as T-sections. Thestiffener attachments must transfer shear loads from the skin to the webportion of the stiffener, as well as out-of-plane loads due to peel(delamination) forces and frame attachments. The web portion of thestiffener extends to a flange portion, which is connected to the skin.Interposed between the web portion and the flange portion is a radiusportion, which must transfer structural loads from the stiffener to theskin material. The current practice is to either co-cure, adhesivelybond, or mechanically fasten the stiffener to the skin. The co-curingand adhesive bonding techniques offer the minimum weight solution.However, the reliability of co-cured and adhesive bonded joints isgenerally low. Mechanically fastened joints (e.g. bolts and rivets) havebeen successfully used, but the use of mechanical joints requires thatthe laminate be reinforced so that it can react to bolt bearing loads.Additionally, the fasteners themselves are both heavy and expensive andthe cost of installation and inspection results in a high cost perfastener.

In addition to bonding techniques, the stiffeners require substantiallyuniform physical properties to function reliably. The transition betweenthe web portion and the skin of the stiffeners can be problematic,especially for CMCs. Of special concern is a fill-it, which is theregion defined by the skin, the radius region(s) and the web portion ofthe T-section. The fill-it is typically filled with the same matrixmaterial used to separate the composite fibers in the plies. For carbonepoxy composites, this works well, as the matrix material, typically apolymeric composition, is substantially as strong as the fibers.However, the same is not true for CMCs, wherein the matrix material issubstantially weaker than the ceramic fibers. Another concern is thatthe fill-it material for CMCs is exposed to higher fabricationtemperatures, such as sintering temperatures wherein ceramic particlesare fired to a temperature just below the melting or fusion point tobond the particles together to form a high strength mass. Due to theresulting increase in density of the bonded particles, there is thepossibility of separation between the fill-it and the adjacent ceramicplies, creating a structurally weak area.

Instead of the matrix material, CMCs typically use pre-impregnated(“prepreg”) ceramic tows or cloth to form the fill-it. A ceramic fibertow or cloth is infiltrated with a ceramic-yielding slurry, whichprepreg tows are cut and individually built up to form the fill-it.Unfortunately, there is no consistent procedure for inserting theceramic tows, and the number of prepreg tows that are used in thefill-it is based on art and practice. If too few prepreg tows are used,then the radius portion of the stiffener is too sharp, which result inregions of stress concentration. However, if too many tows are used inthe fill-it, then a bulge in the radius may occur, resulting in poorstructural properties in the radius portions.

Therefore, what is needed is a composition for use in the fill-it thatis inexpensive to make, easy to form and apply in a consistent manner,which provides substantially consistent physical properties in CMCstiffeners.

SUMMARY OF THE INVENTION

One embodiment of the present invention is directed to a composition foruse with fabricating a ceramic composite stiffener including a webportion, at least one flange portion, a radius region disposed betweenthe web portion and the at least one flange portion, a skin member thatis secured to the at least one flange portion and the radius region. Thecomposition is applied along the radius region adjacent the skin memberand includes refractory particles, plasticizers, and silica-yieldingpolymers. A pliable composition is formed by mixing the refractoryparticles, the plasticizers, the silica-yielding liquids and solvent.The evaporable solvent comprises the balance of the mixture and is addedto achieve the desired consistency. Since the plasticizers,silica-yielding liquids and solvent may or may not be comprised of asingle constituent material, these terms as used herein may be referredto in either the singular or plural form.

An alternate embodiment of the present invention is directed to a methodfor fabricating an improved ceramic composite stiffener including a webportion, at least one flange portion, a radius region disposed betweenthe web portion and the at least one flange portion, a skin member thatis secured to the at least one flange portion and the radius region. Thesteps include preparing a pliable composition, which includes refractoryparticles, plasticizers, and silica-yielding liquids and applying thecomposition along the radius region adjacent the skin member prior toassembling the skin member. An evaporable solvent is added to achievethe desired consistency for the composition.

One advantage of the composition of the present invention is that it isinexpensive to make.

Another advantage of the composition of the present invention is that itis easy to form and apply in a consistent manner. The evaporable solventcan be added as required to achieve the desired consistency.

A further advantage of the composition is that it provides substantiallysimilar physical properties in CMC stiffeners to the CMCs comprising theskin and flange sections. It can be applied as a filler in the radiusregion adjacent the skin member and flange without producing a bulgefrom too large a radius or a region of stress concentration from toosharp of a radius.

A still further advantage of the composition is that, if properlystored, it can be reused. It can thus be premixed and stored until itready to be applied.

Other features and advantages of the present invention will be apparentfrom the following more detailed description of the preferredembodiment, taken in conjunction with the accompanying drawings whichillustrate, by way of example, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section of an improved ceramic matrix compositestiffener of the present invention.

FIG. 2 is a stiffener configuration with an insufficiently sizedfill-it.

FIG. 3 is a stiffener configuration with an excessively large fill-it.

FIG. 4 is an embodiment having a cylindrically shaped profile of apliable composition configured for use as a stiffener fill-it of thepresent invention.

FIG. 5 is a cross section of a stiffener in partial assembly with thecylindrically shaped profile of FIG. 4 installed in the presentinvention.

FIG. 6 is the cross section of the partially assembled stiffener of FIG.5 after the cylindrically shaped profile of FIG. 4 has been shaped intoa fill-it of the present invention.

Wherever possible, the same reference numbers will be used throughoutthe drawings to refer to the same or like parts.

DETAILED DESCRIPTION OF THE INVENTION

A typical stiffener construction to which the invention can be appliedis illustrated, by means of example, in FIG. 1. As shown, a stiffener 10includes a ply 12 constructed of ceramic fiber and matrix materialhaving a flange portion 16. A second ply 14 similarly constructed ofceramic fiber and matrix material also has an adjacent parallel flangeportion 16 lying in substantially the same plane. The flange portions 16are preferably substantially coplanar to receive a skin member 22, alsoreferred to as a hat. Each of flange portions 16 transition to acorresponding radius portion 20, here an internal radius, which furthertransitions to a web portion 18, also referred to as a stiffener, thatis typically substantially transverse to skin member 22. Web portions 18of plies 12 and 14 are opposed to each other and are bonded together toprovide out-of-plane structural strength from flange portions 16. Thetransition from web portions 18 to radius regions 20 of plies 12, 14define a V-groove 23. The enclosed region defined by web portions 18,V-grooves 23 of radius regions 20 and skin member 22 defines a region 25that is a void, which resembles a triangle with opposed sagging sides.This void is provided with a fill-it 24, which provides structuralsupport by transferring structural loads from stiffener 10 to skinmember 22.

Although stiffener 10 is often referred to as a T-section, it is notnecessary that web portions 18 and flange portions 16 are perpendicular.It is also possible that stiffener 10 can have a single ply 12 and askin member 22 such that the fill-it 24 has only one radius region 20.Further, it is possible that stiffener 10 can have more than a singleweb portion 18. While stiffeners 10 typically have multiple plies orlayers, for clarity, flange portions 16 are shown comprising singlesubstantially planar plies 12, 14 and a single hat or skin member 22. Itwill be understood by those skilled in the art that each stiffenermember can include a plurality of plies, the plies added as needed tosatisfy load requirements.

Referring to FIGS. 1-3, fill-it 24 is added to region 25 and ispreferably sized to provide structural support to each radius region 20.Sized properly, fill-it 24 provides a smooth, seamless transitionbetween adjacent flange portions 16 along V-groove 23 to bond skinmember 22. Further, the opposed sagging sides of region 25 permitfill-it 24 to provide radius regions 20 to be at least about 0.030inches. However, providing an insufficient amount of fill-it 24 material(FIG. 2) likewise produces a radius region 20 that is less than 0.030inches, resulting in an area of raised stress concentration. Conversely,applying too much fill-it 24 material along V-groove 23 (FIG. 3) resultsin a bulge 26 along each of the radii of the radius region 20, and anadditional bulge 26 along the skin member 22. Each bulge 26 is adiscontinuity from the otherwise smooth flow of stiffener material,which interferes with the ability of the stiffener to transferstructural loads from one axis to another. That is, from the directionof plies in the web portion 18, which defines a first axis, to thedirection of plies in the flange portion 16, that defines a second axis.The known art fill-it 24 material is comprised of prepreg ceramic towsthat are cut and individually placed along the V-groove 23 and suffersfrom the infirmity of insufficient or excessive application of prepregceramic tows are often applied along the V-groove 23 of the stiffener 10(FIGS. 2-3). Even when applied properly, it is time consuming andexpensive to arrange the prepreg tows and properly place and align them.Such application can significantly compromise the strength and servicelife of the stiffener 10.

A composition of the present invention is provided for application alongthe radius portion adjacent the skin member into region 25 and form thefill-it 24. Stated another way, where there are opposed flange portions16 transitioning to opposed radius regions 20, which opposed radiusregions 20 define the V-groove 23, the composition may be applied alongthe V-groove 23 into region 25 to form the fill-it 24. Referring toFIGS. 4-6, the composition comprises a mixture of ceramic particles,plasticizers, silica-yielding liquid and solvents to form aceramic-yielding putty that may be automatically or manually mixed. In apreferred embodiment, the putty is formed into a substantially pliablecylindrical shape 28 that is about one eighth of an inch in diameter.

The ceramic particles may include alumina, crushed CMCs, calcined claysand grog (crushed refractory brick or glass) or a combination thereof.The ceramic particles are provided as filler to reduce shrinkage of thefill-it during sintering of the stiffener as well as add strength to thefill-it by promoting adhesion with the silica that is formed from thesilica-yielding liquid. The selection of the material used as theceramic particles is based on matching its thermal expansion with theceramic plies in the stiffener. Preferably, the ceramic particlesconsist of both large particles, in the size range from −20 mesh to +50mesh, and small particles, in the size range of −50 mesh. Preferably,the ceramic particles represent from about 55 percent to about 72percent of the mixture by weight (exclusive of solvent), depending uponthe application, to achieve the desired results.

The plasticizers are organic materials, but preferably are typicallycomprised of 2 parts by weight BUTVAR®B76 or B79, which is registeredtrademark of Solutia Inc. of St. Louis, Mo., to 1 part by weight dibutylphthalate and are used to help maintain the composition in a putty-likeconsistency during application. A range of from about 1 percent to about3 percent by weight of plasticizers is normally required in thecomposition (exclusive of solvent) to achieve the desired results.

The silica-yielding liquids are typically comprised of non-curingsilicones, such as sold by General Electric under the names of TPR-179and SR80M and are added to the composition to bond the ceramic particlesof the fill-it 24 together during and after sintering. Similarly, thesilica-yielding liquids also bond the skin member plies 12, 14 and theV-grooves 23 together during and after sintering. A range of from about20 percent to about 26 percent by weight is normally required in thecomposition (exclusive of solvent) to achieve the desired results.

Evaporable solvents, preferably alcohol, and most preferably isopropanoland ethanol, are used to dissolve the BUTVAR®B76 or B79 plasticizer aswell as wet the ceramic particles. However, other solvents such asacetone, may be used. A range of from about 2 percent to about 10percent by weight is normally required in the composition to achieve thedesired results. Since the solvent evaporates during themixing/installation process, that is, is not present in the final, curedstate after sintering, the weight percentage of the solvent is notincluded as part of the weight percentages of the ceramic particles,plasticizers or silica-yielding liquids. Stated another way, if the sumof the ceramic particles, plasticizers or silica-yielding polymersequals ten pounds, a 10 percent weight concentration of solvent simplymeans that 1 pound of solvent would then be added to the existing 10pounds of the ceramic particles, plasticizers or silica-yieldingliquids.

To create the composition, the ceramic particles were slowly added to amixture of the plasticizers, silica-yielding liquids and solvents.Similar to preparing bread dough, this mixture was then kneaded until aputty-like consistency was achieved. The proportion of the ceramicparticles added to the solution varied from “batch to batch,” dependingupon humidity, temperature or other environmental factors, as well asthe desired application, so subjective aspects of each batch werenecessarily evaluated. However, it was observed that once a sufficientquantity of ceramic particles had been added to achieve a sufficientlypliable mixture of substantially uniform consistency that wassubstantially, if not completely cohesive, and not adhesive, a workableputty mixture had been prepared. In other words, if the mixture wassubstantially of uniform consistency and substantially no longer adheredto the mixing instruments (or the hands (gloves) of the individualmixing the batch), a workable batch of the putty had been achieved. Theterm uniform consistency refers to the putty material that is a mixturehaving a substantially uniform distribution of suspended ceramicparticles, which can be rolled into a thin, cylindrical shape withoutbreaking. If the composition was kept sealed in a container from whichthe solvent cannot evaporate, the composition was saved, if desired, andreused. However, the putty hardened if the container was left open andthe putty exposed to air for several hours by evaporation of thesolvents.

Referring to FIGS. 4-6, the pliable, putty-like composition forproviding the fill-it of the present invention may be formed into asubstantially pliable cylindrical shape 28 resembling a noodle having adiameter of about one eighth of an inch. Since the composition is easyto work, a user may shape noodle 28 manually, or an automated shaping orextruding process may be used to produce the noodle. It may be desirableto fabricate wrapped prefabricated lengths of noodle 28 for use asneeded, although the mixture of the composition, if properly contained,may be reused. Alternately, it may be desirable to form the compositioninto a shape that more closely resembles a triangle, such as byextruding, since this shape more closely resembles the final shape ofthe fill-it.

To install the composition in a stiffener, referring to FIG. 5, a userneed merely direct a length of noodle 28 along a corresponding length ofV-groove 23 of a partially assembled stiffener 10, which at the time ofadding noodle 28, lacks skin member 22. After noodle 28 has beeninstalled along V-groove 23 between adjacent flange portions 16, theuser may employ a forming tool 30 having the required diameter orspatula to press a portion of the noodle 28 into the base of theV-groove 23, substantially fill the V-groove, and to form the remainingportion of the noodle material so that it is substantially flush withthe surfaces of the flange portions 16. Preferably, forming tool 30 alsois configured to remove excess noodle material, if any, from thestiffener. After noodle 28 has been installed and shaped, it is allowedto dry and cure. Upon the addition of skin member 22, the assembly ofstiffener 10 is complete (FIG. 1), although subsequent manufacturingsteps remain.

Once the skin member 22 is placed in contact with flange portions 16,and the noodle is allowed to dry, typically the stiffener 10 is vacuumbagged, that is, it is placed in a vacuum, such as by placing thestiffener 10 in a pliable container, such as plastic sheet, and removingthe air contained within the pliable container. By removing the air, thestiffener components are brought into intimate contact, commonlyreferred to as debulking the plies. After the stiffener has beendebulked, it is then placed in an autoclave which subjects the stiffenerto nominally 200 psi pressure and about 300° F. which bonds and curesthe ceramic plies as well as hardens the fill-it and bonds (by adhesion)it to the ceramic plies. After autoclaving, the stiffener is subjectedto a nominal 1,800° F. for a 4 hour sintering operation, during whichany organics typically are removed and the silica-yielding liquidsdecompose to silica which bonds the ceramic particles together as wellas bonds (through ceramic interactions) the fill-it to the ceramicplies. Once sintering is complete, the resulting stiffener with afill-it that has similar physical properties to the ceramic plies isready for structural use.

While the invention has been described with reference to a preferredembodiment, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims.

1. A ceramic composite stiffener comprising: a web portion; at least oneflange portion; a radius region disposed between the web portion and theat least one flange portion; a skin member secured to the at least oneflange portion and the radius region, the composition being appliedalong the radius region adjacent the skin member, the compositioncomprising: (a) about 55% to 72% ceramic particles; (b) about 1% to 3%plasticizers; and (c) about 20% to 26% silica-yielding liquids; andsufficient solvent to permit mixing of the components and forming apliable composition.
 2. A method for fabricating an improved ceramiccomposite stiffener including a web portion, at least one flangeportion, a radius region disposed between the web portion and the atleast one flange portion, a skin member that is secured to the at leastone flange portion and the radius region, the steps comprising:preparing a pliable composition comprising ceramic particles,plasticizers, and silica-yielding liquids, and sufficient solvent formixing of the components to form the pliable composition; applying thecomposition along the radius region adjacent the skin member prior toassembling the skin member; drying the composition; autoclaving thestiffener; and sintering the stiffener.
 3. The method of claim 12wherein the step of sintering the stiffener includes sintering thestiffener at about 1,800° F.
 4. The method of claim 12 further includingan additional step, after the step of drying the composition, ofdebulking the stiffener.
 5. The method of claim 12 wherein the step ofpreparing the composition comprises adding the ceramic particles to asolution containing a mixture of plasticizers, silica-yielding liquidsand solvents to form the pliable composition.
 6. The method of claim 12wherein the pliable composition is a putty-like composition.
 7. Themethod of claim 12 wherein the pliable composition is substantiallycohesive.
 8. The method of claim 12 wherein the pliable composition isof uniform consistency.
 9. A method for fabricating an improved ceramiccomposite stiffener including a web portion, at least one flangeportion, a radius region disposed between the web portion and the atleast one flange portion, a skin member that is secured to the at leastone flange portion and the radius region, the steps comprising:preparing a pliable, putty-like, substantially cohesive compositioncomprising ceramic particles, plasticizers, and silica-yielding liquids,and sufficient solvent to permit mixing of the components and formingthe pliable composition; applying the composition along the radiusregion adjacent the skin member prior to assembling the skin member;drying the composition; debulking the stiffener; autoclaving thestiffener; and sintering the stiffener.